1. Field of the Invention
The present invention relates generally to video capture devices and, more specifically, to a method for automatic exposure control within a video capture device.
2. Description of the Related Art
Digital video camera (DVC) technology has developed rapidly over the past decade. A broad variety of DVCs are now available to meet the diverse needs of a wide spectrum of consumers. DVC technology provides a user with a convenient device that records video and audio and also provides the ability to transfer the recorded video and audio to a computer-readable medium. The computer-readable medium may be, for example, a Digital Video Disc (DVD) or a computer memory.
A DVC typically includes an optical lens assembly configured to focus images from an illuminated environment onto an active region of an image sensor. Because illumination for a given environment may vary significantly, the DVC must be able to adjust to prevailing illumination conditions while still capturing a sequence of properly exposed video frames that comprise a video sequence. A properly exposed frame should reveal the subject or region of interest somewhere in the middle of the dynamic range of the sensor. Exposure gain, which is proportional to exposure time, and analog gain (image sensor gain) are two parameters that conventionally determine how a given video frame is sampled by the DVC. A longer exposure time and higher analog gain may be used in low light settings, while a shorter exposure time and lower analog gain may be used in well-illuminated settings. As sequential frames are captured by the DVC, these two parameters are typically adjusted by the DVC to maintain a sequence of properly exposed video frames.
A DVC user typically records video in uncontrolled environments that include sources of bright light and regions that are relatively dark. For example, a typical apartment room with daylight entering through an unobstructed window may include very bright visual regions around the window and where sunlight strikes objects within the room, and relatively dark visual regions away from the sunlight. When the DVC user pans from a bright visual region to a dark visual region, the DVC employs an automatic exposure means to automatically adjust the exposure gain and analog gain to compensate for the highly variable and quickly changing illumination presented to the image sensor.
In conventional DVC systems, exposure gain and analog gain are discrete, quantized values which are selected to attempt to yield proper exposure for a given image (video frame). When a user pans between light and dark visual regions, quantization granularity of the exposure gain and analog gain can yield a sequence of video frames that appears to become abruptly lighter or darker as the camera adapts to changing lighting conditions. Abrupt changes to image brightness do not typically appear natural and degrade overall video sequence quality. Instead, perceived image brightness should change smoothly as the DVC pans from light to dark visual regions.
One solution for reducing abrupt changes in image brightness involves including a mechanical iris assembly within the optical path of the optical lens assembly. The iris assembly provides certain DVC systems with a primary illumination adjustment parameter that can be used to maintain more constant image brightness over a sequence of images and therefore mitigate abrupt changes to perceived scene brightness. However, an iris assembly can be quite expensive and therefore does not represent an effective solution for many types of DVC systems.
Accordingly, there remains a need in the art for an effective automatic exposure technique that reduces abrupt changes in image brightness within digital video camera systems.